A laser ultrasound method is one technique for non-contact detecting and measuring defects on a surface of and inside an object such as a concrete or steel structure. In the laser ultrasound method, the surface displacement of a target object is measured by generating an elastic wave in the target object, and meanwhile casting a laser beam at the target object and detecting reflected light with a laser interferometer. Because displacements caused by an elastic wave change discontinuously across defects, defects can be detected by measuring the distribution of the displacements. However, since the detection laser (probe laser) of a laser interferometer is spot-like, it is necessary to move the laser spot across (or scan) the entire inspection region of a target object, and a problem is that such scanning takes time.
As an improvement on the laser ultrasound method, an inspection technique is available which uses electronic speckle pattern interferometry as means for measuring surface displacements caused by an elastic wave (see Patent Literature 1). This method targets an object that has a rough surface. A laser beam is expanded by an expander to generate laser light that is cast onto the entire inspection region of the target object. The laser light is scattered at the rough surface, and the scattered laser lights interfere with each other and generate a light and dark pattern which is called a “speckle pattern”. The speckle pattern and reference laser light that was branched from the cast laser light are caused to interfere with each other, and are photographed by a CCD camera or the like. Two such images are taken: one is taken before displacements, and the other is taken after displacements caused by an elastic wave in the object. The distribution of the displacements in the inspection region is then calculated based on the two images. By this means, the displacements in the entire inspection region can be measured at one time.